Transmissible spongiform encephalopathies (TSEs), also known as prion disorders are a group of infectious neurodegenerative diseases affecting humans and animals. The pathological features of TSEs are brain spongiform degeneration, extensive neuronal loss, astrogliosis, and cerebral accumulation of a misfolded and protease-resistant form of the prion protein (PrPres). Compelling evidence indicates that a hallmark event in the disease is the conversion of the normal cellular prion protein (PrPc) into PrPres. Little is known about the mechanism by which this process is associated with the disease and the cellular factors modulating it. We have recently reported that neuronal apoptosis in TSEs is associated with endoplasmic reticulum (ER) stress and activation of the ER-resident caspase-12. Our results indicate that exposition of neuronal cells to nano-Molar concentrations of purified prPres leads to a two-phase response consisting of an initial protective cellular pathway involving the upregulation of specific ER-chaperon proteins (mainly the disulfide-isomerase Grp58), followed by activation of caspase-mediated apoptosis. The in vitro results were supported by in vivo data showing that in a murine scrapie model the first alteration consisted on the up-regulation of Grp58 in the brain, which was detected during the pre-symptomatic phase and followed closely the formation of PrPres. Grp58 upregulation was also detected in the brain of people affected by sporadic and variant CJD, but not in other neurodegenerative disorders. The major goal of this project is to study the role of Grp58 on the pathogenesis of prion diseases and to attempt developing a novel diagnosis test based on early detection of this protein in biological fluids. The specific aims are the following: 1) Determine the onset of neuropathological and clinical alterations in Grp58 transgenic and knock out mice inoculated with scrapie. 2) Evaluate the interaction of Grp58 and PrP and identify compounds inhibiting it. 3) Study the influence of Grp58 on prion propagation in vitro and in scrapie infected neuronal cells and evaluate the effect in prion replication of compounds inducing mild ER-stress. 4) Develop a biochemical diagnosis for TSE based on detecting Grp58 in biological fluids of animals and patients affected by CJD. The findings generated in this project will contribute to understand the molecular basis of prion propagation, neuronal death and brain damage in TSE and will provide novel targets for therapy and diagnosis of these diseases.